The operation of the control clusters implies that the absorbent pencils can be removed entirely outside the core, whereas they are very long, of a length substantially equal to that of the fuel assemblies and without any other connection between them than at their top end where there is the spider. This is what renders indispensable the guidance of the clusters and even of each absorbent pencil individually over the whole length of their movement above the core.
In order to provide this guidance, one conventionally employs tubular housings, also called "guide tubes," which are of sufficient length to cover the whole length from a core plate which defines the upper boundary of the core and which only the absorbent pencils of the control cluster pass through in order to penetrate into the fuel assemblies, up to an upper support plate which carries the weight of the guide tubes.
As they thus form part of what is called the upper internal equipment of the reactor, the guide housings are subjected to the primary cooling fluid which leaves the fuel assemblies vertically at the top end of the core in order then to be directed towards the outlets at the side through which it leaves the vessel which contains the core. Hence they are situated in a zone of great turbulence. Consequently, not only must the housings ensure effective guidance of the absorbent pencils so that they may be introduced, at the time of an emergency shutdown, for example, as rapidly as possible between the fuel pencils, but in addition they must satisfy severe demands upon their mechanical strength. They must in particular be resistant in torsion and in bending; in short, they must withstand large radial and vibratory forces which result from the turbulence of the primary cooling fluid, as well as possible forces due to seismic shocks. They must also be able to withstand abrupt variations in pressure without deformation, especially in the event of rupture of pipework in the primary circuit. Again, they participate in the discharge and the deflection of the primary fluid which enters the interior through the orifices for passing through the absorbent pencils of the control cluster, and they must do so while causing the least possible loss of pressure.
As a function of these essentials, it has already been proposed to reinforce the individual guidance of the absorbent pencils close to the bottom end of the housings. In one known realization, described especially in French Pat. No. 2.397.043, a guide device is employed, which comprises besides the housings themselves and inside each of them horizontal crossbars uniformly distributed over the height of the housing, with in addition in the bottom zone of the housing a set of split vertical tubes and vertical plates directed radially which join horizontal crossbars together so as to ensure longitudinal guidance which is continuous in this zone, while letting the absorbent pencils and even the spider pass freely. The longitudinal slits which enable the spider to pass through considerably weaken the mechanical strength of the tubes. In addition their performance is limited by the existence of a pressure gradient in a horizontal plane which, because the tubes are open, tends to apply the absorbent pencil against the wall of the tube, thereby causing wear by friction. In order to limit this phenomenon, openings for discharge of the fluid are provided in the radial plates arranged between the tubes.
In accordance with another solution (French Pat. No. 73-23490, published under the No. 2,202,339), the top plate of the core is doubled by an intermediate plate situated at a level higher than the region of strongest turbulence of the primary fluid, in practice above the plane of the pipe stubs in the vessel containing the core, which provide the inlet and outlet of the cooling fluid. The intermediate plate is not traversed by the housings, but only by tubes which provide the individual guidance of the absorbent pencils at the same time as they play the part of crosspieces holding the two plates (the core plate and the intermediate plate) solidly together. Hence in this case the housings themselves remain outside the zone of strongest turbulence, as does the portion comprising the spider and the operating rod for the control rods. This solution is, however, only possible with tubes of relatively large diameter, capable of displaying the mechanical strength indispensable for their part of cross pieces. Hence it is not applicable to the guidance of control rods having absorbent pencils of small diameter. This disadvantage is not completely avoided in a variant upon this solution described in French Patent Application No 73-44815, published under No. 2,254,859, in which the intermediate plate is lowered and equipped with orifices for passing through the cooling fluid, so that the circulation remains substantially a rising one at this level.
In order to alleviate the disadvantages of the various known guide devices recalled above, the invention proposes a new device which has the advantage of enabling the combination especially of great rigidity, less sharing of the absorbent pencils in the wear and less elaborate manufacture.